/* An instance of a concrete descendant of this class represents an expression
** (perhaps arithmetic, or boolean).
**
** Author: R. McCloskey
** Date: Feb. 2022
*/
public abstract class ExprTree {

   // class constants
   // ---------------
   public static final int PrefixMode = 0; 
   public static final int InfixMode = 1;
   public static final int PostfixMode = 2;

   // instance variable
   // -----------------
  
   // The value of this variable (which is necessarily one among the three 
   // "print mode values" defined above) dictates the form of the String
   // produced by the toString() method.
   private int printMode;
                         

   // constructor
   // -----------

   /* Initializes this expression to be in the specified print mode.
   */
   public ExprTree(int printMode) {
      this.printMode = printMode;
   }

   /* Initializes this expression to be in infix print mode.
   */
   public ExprTree() { this(InfixMode); }

   // observers
   // ---------

   /* Returns the value of this expression.
   */
   public abstract int valueOf();

   /* Returns this expression's current print mode value.
   */
   protected int printMode() { return printMode; }

   /* Returns a String describing this expression in the form specified
   ** by the parameter, whose value is expected to be one of the "print
   ** mode" values defined above.
   */
   public abstract String toString(int mode);

   /* Returns a String describing this expression in the form corresponding
   ** to this expression's current print mode value.
   */
   @Override
   public String toString() { return toString(printMode()); }


   // mutators
   // --------

   // Methods by which to set this expression's print mode.

   public void setToPrefixMode() { printMode = PrefixMode; }
   public void setToInfixMode() { printMode = InfixMode; }
   public void setToPostfixMode() { printMode = PostfixMode; }

}